1. Field of the Invention
The invention relates to a method for producing an injection valve for liquids, preferably an injection valve for injecting fuel into a combustion chamber of an internal combustion engine, and a device for carrying out the method.
2. Description of the Prior Art
A method for producing an injection valve is known for instance from European Patent EP 233 190 B1. Injection valves of the kind preferably used for injecting fuel at high pressure into a combustion chamber of an internal combustion engine are produced in a plurality of work steps. In a first work step, the actual valve body is shaped from steel, the steel being still unhardened and thus easily machined. After the production of the outer and inner shape of the injection valve, the valve is subjected to a hardening process, in which the injection valve is severely heated and then quenched. For eliminating stresses that are caused by the uneven cooling down of the valve body, the hardening is followed by a heat treatment of the valve body, that is, a first annealing, in which the valve body is held for a certain period of time at an elevated temperature below the hardening temperature, until the stresses that have built up are adequately reduced. The valve body can be heated by various methods, for instance in an oil bath or by inductive heating, which is known for instance from German published patent application DE-OS 1 292 696.
Injection valves of the kind used for injecting fuel at high pressure into combustion chambers of internal combustion engines operate in the following way: A valve needle is disposed longitudinally displaceably in the valve body and cooperates with a valve seat for opening and closing at least one injection opening. By a longitudinal motion of the valve needle, the injection is initiated and interrupted, so that a precise injection of fuel at high pressure into the combustion chamber is attained. This puts a severe strain on the valve needle in the region where it is guided and on the valve seat, but wear is lessened by the aforementioned hardening of the valve body.
In modern valve bodies, upon their first use once engine operation has begun, a partial transformation of the microstructure of the material takes place, which leads to a change in shape in the nozzle seat at the place where the valve needle is seated on the nozzle. This change in shape causes a change in the quantity injected, so that the injection valve no longer operates as it did when it was new, which is especially disadvantageous in common rail injection systems.
It is not possible to optimize the valve body in every respect: Longer annealing at higher temperatures does increase the toughness of the steel but also lessens its hardness, so that the wear in the region of the valve seat and of the needle guidance becomes greater. On the other hand, if the annealing is done at lower temperatures or for a shorter time, then the valve body remains relatively brittle, and cracks can occur, particularly in the region of the valve seat.